Features: Add infrastructure for C++ 20 language standard

Issue: #17849

7 files changed, 27 insertions, 2 deletions

diff --git a/Modules/CMakeCUDACompilerId.cu.in b/Modules/CMakeCUDACompilerId.cu.in
index 018bab7..6eda924 100644
--- a/Modules/CMakeCUDACompilerId.cu.in
+++ b/Modules/CMakeCUDACompilerId.cu.in
@@ -17,7 +17,9 @@ char const* info_simulate = "INFO" ":" "simulate[" SIMULATE_ID "]";
 @CMAKE_CUDA_COMPILER_ID_ERROR_FOR_TEST@
 
 const char* info_language_dialect_default = "INFO" ":" "dialect_default["
-#if __cplusplus > 201402L
+#if __cplusplus > 201703L
+  "20"
+#elif __cplusplus >= 201703L
   "17"
 #elif __cplusplus >= 201402L
   "14"
diff --git a/Modules/CMakeCXXCompiler.cmake.in b/Modules/CMakeCXXCompiler.cmake.in
index df57a4f..974886d 100644
--- a/Modules/CMakeCXXCompiler.cmake.in
+++ b/Modules/CMakeCXXCompiler.cmake.in
@@ -10,6 +10,7 @@ set(CMAKE_CXX98_COMPILE_FEATURES "@CMAKE_CXX98_COMPILE_FEATURES@")
 set(CMAKE_CXX11_COMPILE_FEATURES "@CMAKE_CXX11_COMPILE_FEATURES@")
 set(CMAKE_CXX14_COMPILE_FEATURES "@CMAKE_CXX14_COMPILE_FEATURES@")
 set(CMAKE_CXX17_COMPILE_FEATURES "@CMAKE_CXX17_COMPILE_FEATURES@")
+set(CMAKE_CXX20_COMPILE_FEATURES "@CMAKE_CXX20_COMPILE_FEATURES@")
 
 set(CMAKE_CXX_PLATFORM_ID "@CMAKE_CXX_PLATFORM_ID@")
 set(CMAKE_CXX_SIMULATE_ID "@CMAKE_CXX_SIMULATE_ID@")
diff --git a/Modules/CMakeCXXCompilerId.cpp.in b/Modules/CMakeCXXCompilerId.cpp.in
index 4cb2267..34639b4 100644
--- a/Modules/CMakeCXXCompilerId.cpp.in
+++ b/Modules/CMakeCXXCompilerId.cpp.in
@@ -34,7 +34,9 @@ char const *info_cray = "INFO" ":" "compiler_wrapper[CrayPrgEnv]";
 #endif
 
 const char* info_language_dialect_default = "INFO" ":" "dialect_default["
-#if CXX_STD > 201402L
+#if CXX_STD > 201703L
+  "20"
+#elif CXX_STD >= 201703L
   "17"
 #elif CXX_STD >= 201402L
   "14"
diff --git a/Modules/CMakeDetermineCompileFeatures.cmake b/Modules/CMakeDetermineCompileFeatures.cmake
index 3ed92be..01a81a1 100644
--- a/Modules/CMakeDetermineCompileFeatures.cmake
+++ b/Modules/CMakeDetermineCompileFeatures.cmake
@@ -49,6 +49,7 @@ function(cmake_determine_compile_features lang)
     set(CMAKE_CXX11_COMPILE_FEATURES)
     set(CMAKE_CXX14_COMPILE_FEATURES)
     set(CMAKE_CXX17_COMPILE_FEATURES)
+    set(CMAKE_CXX20_COMPILE_FEATURES)
 
     include("${CMAKE_ROOT}/Modules/Internal/FeatureTesting.cmake")
 
@@ -59,6 +60,9 @@ function(cmake_determine_compile_features lang)
       return()
     endif()
 
+    if (CMAKE_CXX17_COMPILE_FEATURES AND CMAKE_CXX20_COMPILE_FEATURES)
+      list(REMOVE_ITEM CMAKE_CXX20_COMPILE_FEATURES ${CMAKE_CXX17_COMPILE_FEATURES})
+    endif()
     if (CMAKE_CXX14_COMPILE_FEATURES AND CMAKE_CXX17_COMPILE_FEATURES)
       list(REMOVE_ITEM CMAKE_CXX17_COMPILE_FEATURES ${CMAKE_CXX14_COMPILE_FEATURES})
     endif()
@@ -75,6 +79,7 @@ function(cmake_determine_compile_features lang)
         ${CMAKE_CXX11_COMPILE_FEATURES}
         ${CMAKE_CXX14_COMPILE_FEATURES}
         ${CMAKE_CXX17_COMPILE_FEATURES}
+        ${CMAKE_CXX20_COMPILE_FEATURES}
       )
     endif()
 
@@ -83,6 +88,7 @@ function(cmake_determine_compile_features lang)
     set(CMAKE_CXX11_COMPILE_FEATURES ${CMAKE_CXX11_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX14_COMPILE_FEATURES ${CMAKE_CXX14_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX17_COMPILE_FEATURES ${CMAKE_CXX17_COMPILE_FEATURES} PARENT_SCOPE)
+    set(CMAKE_CXX20_COMPILE_FEATURES ${CMAKE_CXX20_COMPILE_FEATURES} PARENT_SCOPE)
 
     message(STATUS "Detecting ${lang} compile features - done")
   endif()
diff --git a/Modules/Compiler/CMakeCommonCompilerMacros.cmake b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
index 684fd30..ad464c7 100644
--- a/Modules/Compiler/CMakeCommonCompilerMacros.cmake
+++ b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
@@ -78,6 +78,9 @@ endmacro()
 # Define to allow compile features to be automatically determined
 macro(cmake_record_cxx_compile_features)
   set(_result 0)
+  if(_result EQUAL 0 AND DEFINED CMAKE_CXX20_STANDARD_COMPILE_OPTION)
+    _record_compiler_features_cxx(20)
+  endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX17_STANDARD_COMPILE_OPTION)
     _record_compiler_features_cxx(17)
   endif()
diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 8a7d64a..8e8ab4f 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -49,6 +49,8 @@ else()
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "")
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "")
 endif()
 
 if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
diff --git a/Modules/Compiler/MSVC-CXX.cmake b/Modules/Compiler/MSVC-CXX.cmake
index 789fff5..6a0c5d3 100644
--- a/Modules/Compiler/MSVC-CXX.cmake
+++ b/Modules/Compiler/MSVC-CXX.cmake
@@ -29,6 +29,12 @@ if ((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.0.24215.1 AND
   # for meta-features for C++14 and above.  Override the default macro
   # to avoid doing unnecessary work.
   macro(cmake_record_cxx_compile_features)
+    if (DEFINED CMAKE_CXX20_STANDARD_COMPILE_OPTION)
+      list(APPEND CMAKE_CXX20_COMPILE_FEATURES cxx_std_20)
+    endif()
+    # The main cmake_record_cxx_compile_features macro makes all
+    # these conditional on CMAKE_CXX##_STANDARD_COMPILE_OPTION,
+    # but we can skip the conditions because we set them above.
     list(APPEND CMAKE_CXX17_COMPILE_FEATURES cxx_std_17)
     list(APPEND CMAKE_CXX14_COMPILE_FEATURES cxx_std_14)
     list(APPEND CMAKE_CXX98_COMPILE_FEATURES cxx_std_11) # no flag needed for 11
@@ -46,6 +52,8 @@ elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "")
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "")
 
   # There is no meaningful default for this
   set(CMAKE_CXX_STANDARD_DEFAULT "")
@@ -60,6 +68,7 @@ elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
       cxx_std_11
       cxx_std_14
       cxx_std_17
+      cxx_std_20
       )
     _record_compiler_features(CXX "" CMAKE_CXX_COMPILE_FEATURES)
   endmacro()